1. Field of the Invention
The present invention relates to apparatuses and methods for reproducing original data from signals detected from recording media, signals supplied from transmitting media, and the like, and in particular, relates to an apparatus and a method for decoding detected signals by maximum-likelihood decoding.
2. Description of the Related Art
In general, apparatuses used for recording and reproducing data to/from a variety of recording media include, for example, detecting units detecting signals recorded on these recording media and decoding units decoding the detected signals from the detecting units to reproduce original binary data.
In recording and reproducing data to/from recording media, in view of increasing recording density, highly-reliable decoding units are disclosed in, for example, Japanese Patent No. 3016366 and U.S. Patent Application Publication No. 2003/0026028. These decoding units decode detected signals to reproduce binary data by a decoding method called the partial response maximum likelihood (PRML) method.
The PRML method includes a technique called the partial response technique for reproducing data strings though a channel having a predetermined impulse response and a technique called the maximum-likelihood decoding technique for selecting data strings presumed to be original data strings out of all possible data strings to decode the selected data strings.
An optical disk playback apparatus irradiates tracks on a disk surface with a laser beam and receives a reflected laser beam through a photodetector to detect data recorded on tracks.
FIG. 4 is a schematic view illustrating a photodetector and a laser spot on a track irradiated with a laser beam, in a known optical disk playback apparatus.
As shown in FIG. 4, a photodetector 100 includes a detecting area composed of four detecting segments A, B, C, and D that are arranged into two vertically and two horizontally. An optical system in an optical disk playback apparatus in use is adjusted so that the center of a laser spot 120 on a track 110 coincides with the center of the photodetector 100. Data recorded within the laser spot 120 is read out to reproduce signals.
The detecting area of the photodetector 100 is separated into four segments mainly because the laser spot needs to be focused on the disk surface. Data is reproduced using a sum signal (also referred to as a pull-in signal) of signals from the detecting segments A, B, C, and D. Thus, separating the detecting area into segments has no special significance for data reproduction.
Signals reproduced in this way are digitized to be input to a maximum-likelihood decoding unit. The maximum-likelihood decoding unit estimates original data based on the reproduced signals. In this way, the original data is reproduced.
In an optical disk playback apparatus having the structure as described above, noise generated in, for example, a photodetector, needs to be reduced so that a maximum-likelihood decoding unit can correctly estimate original data. A possible way to reduce the influence of noise generated in the photodetector is to reproduce the same data multiple times. However, the optical disk playback apparatus does not reproduce the same data multiple times. Thus, the same data is preferably reproduced through multiple channels at the same time.
A plurality of signals can be reproduced using the above-described known photodetector including the detecting area composed of the separated detecting segments. That is, reproducing a plurality of signals using such a detecting area corresponds to reproducing the same signal multiple times.
However, the intensity of each of the plurality of reproduced signals in each segment is disadvantageously reduced, and there is a problem due to the structure of a known maximum-likelihood decoding unit.
Although the above-described photodetector including the detecting area composed of the separated detecting segments can reproduce a plurality of signals at the same time, the level of a signal output from each detecting segment is low relative to noise, so that the advantage provided by the separated detecting segments is offset by the low signal level.
Since the known maximum-likelihood decoding unit is originally designed so as to use only a reproduced signal to estimate recorded original data, only a sum signal, or a difference signal, at best, from a plurality of reproduced signals can be decoded by maximum-likelihood decoding even when there are a plurality of detecting segments.
Thus, it is required that a plurality of signals be generated without decreasing the signal intensity and that a maximum-likelihood decoding unit estimate original data using multiple inputs.